The stomach of the cow is a remarkable organ. It has four compartments
(rumen, reticulum, omasum, and abomasum), each designed to perform certain
functions. By having this stomach, the cow is able to digest and efficiently
use feedstuffs that humans and non-ruminant animals, such as the pig and
chicken, cannot use. Examples of these feedstuffs include cellulose, which
is used for energy, and urea, which is used to make protein.

When a calf is born the forestomach compartments (rumen, reticulum and
omasum) are not developed, therefore the calf is unable to utilize dry
feed. Development includes growth in size and musculature, growth of the
papillae of the mucosal (inner) surface to provide for absorption of end
products of digestion, and establishment of the microorganisms (bacteria,
protozoa, fungi, etc.) that are responsible for digesting the feedstuffs.
This development occurs only if dry feed is consumed, otherwise the forestomach
compartments remain small and undeveloped.

Because dry feed cannot be used by the very young calf, it depends on
milk for nutrition. As the calf grows it eats a small amount of dry feed
at first, then consumes increasingly larger amounts, and the proportion
of nutrients from milk decreases.

The nutrients in milk are digested in the fourth stomach compartment
(abomasum) and intestines, and it would not be desirable for milk to go
into the forestomach. To make it possible for dry feed to go into the rumen
but for milk to go into the abomasum the calf has what is called the esophageal
groove. This groove is actually two folds of tissue that can be brought
together to form a tube. When the calf drinks milk a reflex action closes
the folds to form a tube which conducts the milk from the end of the esophagus
to the abomasum. When the calf eats dry feed the groove does not close
and the dry feed is directed into the rumen and reticulum.

The above description is actually an over simplification of a complex
system and series of events that take place. It should be obvious that
this system was designed by an all wise Creator and did not happen by chance.
(Editor's Note: Morrill was on the faculty at Kansas State University for
thirty-three years and was involved in research that established some of
the facts in the above article.)